1. Field of the Disclosure
Embodiments disclosed herein relate generally to the conversion of hydrocarbons or hydrocarbon-containing mixtures to ethylene. In another aspect, embodiments disclosed herein relate to the conversion of methane to ethylene. In another aspect, embodiments disclosed herein relate to the selective hydrogenation of acetylene to ethylene.
2. Background
Natural gas typically contains about 60 to 100 mole percent methane, the balance being primarily heavier alkanes. Alkanes of increasing carbon number are normally present in decreasing amounts. Carbon dioxide, hydrogen sulfide, nitrogen, and other gases may also be present in relatively low concentrations.
The economical conversion of methane into valuable reactive hydrocarbon and reactive non-hydrocarbon products has been a technological goal for many years. For example, light olefins, such as ethylene and propylene, serve as the building blocks for the production of numerous chemicals. For example, intermediate and end uses of ethylene include production of plastics, resins and fibers, and a host of other products.
Methane may be converted to ethylene via methanol or dimethyl ether. Methane may be first converted to syngas, which is subsequently converted to methanol or a mixture of dimethyl ether and methanol. The methanol and/or dimethyl ether may then be converted to ethylene in a final step. Production of ethylene from methane via methanol is disclosed in, for example, U.S. Pat. No. 5,811,621.
Methane may also be converted to a mixture of unsaturated hydrocarbons and hydrogen by partial oxidation or non-oxidative pyrolysis, as described at length in U.S. Pat. No. 7,208,647. Such conversion processes may result in a product stream including hydrogen, carbon monoxide, carbon dioxide, water, methane, acetylene, ethylene, and other hydrocarbons. Similar feedstocks may also be produced by cracking or partial oxidation of various hydrocarbon feeds, such as ethane, propane, and other saturated hydrocarbons. U.S. Pat. No. 6,212,905 describes various processes converting methane to ethylene via cracking.
As noted above, acetylene is a by-product for various processes converting methane to ethylene. Acetylene, the simplest alkyne, may be converted to ethylene by hydrogenation. It is typically desirable to convert acetylene to ethylene, but not to convert ethylene to ethane by further hydrogenation, thus resulting in the valuable reactive olefin. The hydrogenation of acetylene to ethylene may be carried out on the raw pyrolysis gas mixture (front-end hydrogenation), or may be carried out following some separation of the various components (tail-end hydrogenation), such as where the only stream subject to hydrogenation is enriched in highly unsaturated compounds (e.g., acetylene).
The advantage of primary gas hydrogenation is generally an abundance of the hydrogen required for hydrogenation. The concentrations of acetylene and hydrogen in the pyrolysis gas mixture are quite high compared with ethane steam cracked products. The mole ratio of hydrogen to acetylene in a pyrolysis gas mixture is approximately 5 to 1. Unfortunately, the excess available hydrogen in front-end hydrogenation can result in “run-away” reactivity, resulting in conversion of alkenes to alkanes and reducing the value of the product. Fractionation reduces the available hydrogen, but polymer formation is common, the effect of which is to shorten the useful life of the catalyst.
As described in EP 01710222, acetylene in the pyrolysis product stream may be selectively hydrogenated to ethylene in the presence of a catalyst in a fixed bed reactor. Various other processes for the hydrogenation of acetylene are described in U.S. Pat. Nos. 7,045,670 and 7,014,750 and U.S. Patent Application Publication No. 20060155154, for example.
Various problems associated with selective hydrogenation of acetylene to ethylene include poor selectivity of ethylene and fast catalyst deactivation due to the formation of undesired “green oil.” Such problems often make the conversion of methane to ethylene unattractive for commercial production of ethylene.
Accordingly, improvements in the selective hydrogenation of acetylene to ethylene are needed.